1. Field of the Invention
The present invention relates to an optical fiber telecommunications network and more particularly, to such a network that provides both narrowband telecommunications signals and broadband switched video signals. The invention provides an optical distribution shelf for use in a remote terminal for distributing optical signals to distribution fibers extending to remotely-located optical network units.
2. Description of the Prior Art
The rapid proliferation of optical fiber telecommunications systems has made possible the provision of broadband services to individual subscribers on a relatively universal basis. Such broadband services often include data transmission; however, there is a broader market for the distribution of video signals over the telecommunications network.
The provision of such video services has long been desired; however, the previously proposed systems have all been subject to various deficiencies which have prevented their commercial acceptance. Video signals may be broadcast to all subscribers over optical fibers; however, this severely limits the programming selection and the number of channels that may be available to each subscriber. A switched video architecture allows for the provision of significantly more programming options and control of distribution only to authorized subscribers.
The concept of switched video transmission systems has been proposed in the past; however, most proposals have had undesirable features. Most proposed switched video architectures require the use of a second optical fiber to distribute the broadband services or, as an alternative, the use of a wavelength division multiplexing system. Such systems do not offer a truly integrated architecture, wherein a single fiber distributes both narrowband and broadband signals and the systems are not integrated with respect to both control and cost effective utilization of common electronics.
The use of two fiber systems to distribute the broadband video service is inefficient in that the second fiber must be installed during the initial deployment of the system when there are extremely high equipment expenses. Currently, regulatory agencies do not always permit recovery of costs associated with a second fiber for broadband services. The use of wavelength division multiplexing significantly complicates a system in that it requires the use of a significant number of fiber couplers which are both high cost and large components. Both alternatives typically double the amount of costly optoelectronics required.
In unidirectional (two-channel) wavelength division multiplexing two distinctly different optical sources and detectors are utilized simultaneously. These sources and detectors must be coupled to the single transmission fiber. Separate transmitters and receivers must accompany each source and detector. This multiplicity of fibers and couplers becomes extremely difficult to handle and requires large and cumbersome equipment or delicate optical arrangements. The need for separate transmitters and receivers for each wavelength makes the cost prohibitive, particularly when universal service is rendered to all customer premises.
An article entitled: "A Future Switched Video System" by John R. Gunter, IEEE LCS Magazine, February, 1990, at page 66 and following, describes the desirability of providing video services over the telecommunications network. Another article entitled: "A High-Quality Switched FM Video System" by David E. Robinson and David Grubb, III, IEEE LCS Magazine, also published February, 1990, at page 53 and following describes a proposed system architecture wherein the various video channels are frequency multiplexed onto a carrier; however, the carrier uses wavelength division multiplexing for upstream and downstream transmissions.
Other articles describing the simultaneous transmission of narrowband and broadband signals are as follows: "A Hybrid Lightwave Transmission System for Subcarrier Multiplexed Video and Digital B-ISDN Services in the Local Loop", by Charles N. Lo, Journal of Lightwave Technology, Vol. 7, No. 11, November 1989, pp. 1839-1848; and "Fiber Optic Analog-Digital Hybrid Signal Transmission Employing Frequency Modulation", by K. Sato et al, IEEE Transactions on Communications, Vol. COM-33, No. 5, May 1985, pp. 433-441.
The upstream and downstream transmission of control information has consistently been a problem in switched video and video on demand systems. Such upstream transmission has been accomplished using wavelength division multiplexing as previously mentioned in the article of David E. Robinson et al; however, such a system is then subjected to the aforementioned deficiencies associated with the use of wavelength division multiplexing. Other systems have used a separate narrowband telephone connection for keying in control data in the upstream direction using a telephone subset. Such systems are not truly integrated and require the use of the premises telephone subset to transmit control information upstream for selection of the desired video.
The RCV-1G system provided by Alcatel used a dedicated FSK subcarrier electrically multiplexed with the narrowband and video for control purposes.
Thus, the prior art has not provided a commercially acceptable architecture for mass deployment of switched video on a telecommunications network.